Modern digital signals, for example over satellite links, may include some kind of framing. Framing may be needed, for instance, in order to determine an end of one code word (e.g. a data frame) and a start of a next code word (e.g. a next data frame) within the transmission. A known sequence of transmission symbols, which may be sometimes referred to as Unique Word (UW), may be used for framing synchronization. For example, in burst transmissions, a single UW (e.g. at the beginning of the burst) may be used for synchronizing the burst framing. In another example, in continuous transmission, a UW sequence may be occasionally transmitted, though for simplifying synchronization UW instances may be periodically transmitted, e.g. at constant intervals or distances (for example measured in symbols).
A UW sequence may be detected using a coherent correlator. The correlator may rotate the known symbols of the UW “backwards”, so that all the UW symbols may be added coherently (i.e. in phase). In some examples, if the phase of the received signal is unknown, an amplitude detector or a power detector may be used at the output of the correlator for detecting the UW existence. However, if a transmitted signal is received at some frequency offset, the UW symbols may be undesirably rotated while being added by a correlator (i.e. due to the frequency offset), causing the said addition of symbols to be at least somewhat incoherent. Thus, this undesired rotation may introduce degradation to the coherent detection, and in severe cases it may prevent detection of the UW. For example, if the transmission symbol rate is denoted as Rs, the UW length in symbols is denoted as L, and the frequency error is denoted as ΔF, a degradation in detection performance (in dB) may be calculated as:20 LOG(SINC(L*ΔF/Rs))wherein LOG is the decimal logarithm function and SINC is the normalized SINC function, defined as:SINC(X)=SIN(πX)/πX wherein SIN is the known trigonometric sinus function.
For example, at a frequency offset of Rs/(4 L), the resulting degradation in detection performance might be in the excess of 1 dB. In another example, at a frequency offset of Rs/L, detection might be entirely impossible (e.g. as ΔF aims to Rs/L, the expression SINC(L*ΔF/Rs) aims to 0 and the expression 20 LOG(SINC(L*ΔF/Rs)) aims to minus infinity, i.e. the degradation at such a frequency offset might exceed the detector capabilities).
Therefore, when trying to determine reception timing of a UW in presence of a relatively large frequency error, a receiver may use one or more techniques in order to enable detection of the UW. One technique is to determine the frequency offset through frequency scanning The receiver may be set to receive the signal at a certain frequency and to try locating the UW. If the UW is not detected, the receiver frequency may be modified to a close neighbor frequency within a preconfigured frequency offset range, where the receiver may try again to locate the UW, and so on until the UW is located or the entire preconfigured frequency offset range may be scanned. However, this scanning method may significantly increase the acquisition time (e.g. the time it may take to synchronize or to lock the receiver on the received signal). It may be noted that if a frequency step size is in the order of Rs/2 L, the residual maximal frequency error is Rs/4 L, hence ensuring up to 1 dB degradation in detection performance. However, selecting a frequency step size in the order of Rs/2 L may imply a relatively large number of frequency steps (e.g. scanning iterations) that may be required before a correct frequency offset may be found.
An alternative method to deal with frequency offset may be to divide the UW correlator into short segments, i.e. to perform the correlation in parts, so that any exiting frequency offset may result in smaller loss of coherency within each segment. The correlation results of all segments may then be further assembled in a non-coherent manner. For example, the amplitude-square of each segment correlation output may be computed and the results from all segments may be then combined. However, a disadvantage of this method may be its use of non-linear operation (e.g. such as squaring) before all fragment contributions may be added. This non-linear operation may introduce another source of detection-loss, often denoted as “Squaring Loss” that may become quite significant, for example in negative SNR (Signal to Noise Ratio) scenarios. The SNR at an output of a short-segment correlation of length m (SNRsegment—out) may be represented as:SNRsegment—out=SNRin+10 LOG(m)wherein SNRin is the SNR of the signal at the input to the correlation. If SNRin is low and the segment length (m) is short, SNRsegment—out may still be low. For example, considering SNRin=−15 dB, UW length L=64, and short segment correlation length m=4. In such example:SNRsegment—out=SNRin+10 LOG(M)=−15+10 LOG(4)=−9 dBSNRfull—correlation—out=SNRsegment—out−Squaring_Loss+10 LOG(L/m)
The squaring loss may be significantly increased as SNRsegment—out becomes more negative (e.g. Squaring_Loss≈SNRsegment—out for negative SNRsegment—out values and at least 3 dB for positive or high SNRsegment—out values). Thus, considering the above example:SNRfull—correlation—out=−9−9+10 LOG(64/4)=−9−9+12=−6 dBHowever, if the frequency offset is negligible, a full correlation may be calculated without segmentation. In such case:SNRfull—correlation—out=SNRin+10 LOG(L)=−15+10 LOG(64)=−15+18=3 dB
Thus, a more efficient technique for synchronizing to a signal framing under a significant frequency offset error may be useful. Such a technique should be fast on one hand and resulting in no or little degradation to detection performance on the other hand.